ABSTRACT
Although most Phytophthora species have a soilborne phase that is crucial for infection of roots and for survival away from the host, the details of the soil phase of Phytophthora ramorum are not yet fully understood. As mycelium ages, it becomes resistant to sterilization by acidic electrolyzed water (AEW), a product of the electrolysis which can be used as a disinfectant. Colonies of P. ramorum could be recovered from moist potting mix or sand for many months, whether buried as infected plant leaf tissue or as mycelium bearing chlamydospores, and the buried material was also resistant to treatment by AEW. There was no significant difference in recovery over time among treatments (sand or potting mix; infected plant tissue or mycelium); after approximately a year, colonies could be recovered at 0.8 to 14.3%. When excised roots were inoculated with P. ramorum sporangia and buried in mesh bags in potting mix, the pathogen was recovered from buried roots for at least 8 to 11 months, but it was not clear whether it was surviving as mycelium or chlamydospores. The roots of living plants of Acer macrophyllum, Buxus sempervirens, Camellia oleifera, C. sinensis, C. sasanqua, Lonicera hispidula, Taxus baccata, Umbellularia californica, Vaccinium macrocarpon, Viburnum davidii, V. tinus, V. × pragense, Rhododendron ‘Gloria’, and Syringa vulgaris were drenched with a sporangial solution of P. ramorum and incubated for a month; the pathogen could be recovered from roots of all plants except those of Buxus sempervirens and Lonicera hispidula. Recovery on selective agar medium (P5ARP) was from both washed and surface-sterilized roots, suggesting that the roots were internally infected. When chlamydospores were placed near roots and observed directly, they were seen to germinate, forming sporangia. Nearby roots became infected, the tips covered with sporangia. Therefore, P. ramorum appears to have a soil phase, at least under greenhouse and nursery conditions.
